1. Field of the Invention
The present invention relates to computer networks and more particularly to calculating a shortest path for Traffic Engineering (TE) label switched paths (LSPs) across autonomous systems of a computer network.
2. Background Information
A computer network is a geographically distributed collection of nodes interconnected by communication links and segments for transporting data between end nodes, such as personal computers and workstations. Many types of networks are available, with the types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect the nodes over dedicated private communications links located in the same general physical location, such as a building or campus. WANs, on the other hand, typically connect geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines, optical lightpaths, synchronous optical networks (SONET), or synchronous digital hierarchy (SDH) links. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes on various networks. The nodes typically communicate over the network by exchanging discrete frames or packets of data according to predefined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP). In this context, a protocol consists of a set of rules defining how the nodes interact with each other. Computer networks may be further interconnected by an intermediate network node, such as a router, to extend the effective “size” of each network.
Since management of interconnected computer networks can prove burdensome, smaller groups of computer networks may be maintained as routing domains or autonomous systems. The networks within an autonomous system (AS) are typically coupled together by conventional “intradomain” routers configured to execute intradomain routing protocols, and are generally subject to a common authority. To improve routing scalability, a service provider (e.g., an ISP) may divide an AS into multiple “areas.” It may be desirable, however, to increase the number of nodes capable of exchanging data; in this case, interdomain routers executing interdomain routing protocols are used to interconnect nodes of the various ASes. Moreover, it may be desirable to interconnect various ASes that operate under different administrative domains. As used herein, an AS is generally referred to as a “domain,” and a router that interconnects different domains is generally referred to as a “border router.”
An example of an interdomain routing protocol is the Border Gateway Protocol version 4 (BGP), which performs routing between domains (ASes) by exchanging routing and reachability information among neighboring interdomain routers of the systems. An adjacency is a relationship formed between selected neighboring (peer) routers for the purpose of exchanging routing information messages and abstracting the network topology. The routing information exchanged by BGP peer routers (BGP speakers or BGP nodes) typically includes destination address prefixes, i.e., the portions of destination addresses used by the routing protocol to render routing (“next hop”) decisions. Examples of such destination addresses include IP version 4 (IPv4) and version 6 (IPv6) addresses. BGP generally operates over a reliable transport protocol, such as TCP, to establish a TCP connection/session. The BGP protocol is well known and generally described in Request for Comments (RFC) 1771, entitled A Border Gateway Protocol 4 (BGP-4), published March 1995, the contents of which are hereby incorporated in its entirety.
An intermediate network node often stores its routing information in a routing table maintained and managed by a routing information base (RIB). The routing table is a searchable data structure in which network addresses are mapped to their associated routing information. However, those skilled in the art will understand that the routing table need not be organized as a table, and alternatively may be another type of searchable data structure. Although the intermediate network node's routing table may be configured with a predetermined set of routing information, the node also may dynamically acquire (“learn”) network routing information as it sends and receives data packets. When a packet is received at the intermediate network node, the packet's destination address may be used to identify a routing table entry containing routing information associated with the received packet. Among other things, the packet's routing information indicates the packet's next-hop address.
To ensure that its routing table contains up-to-date interdomain routing information, the intermediate network node may cooperate with other intermediate nodes to disseminate routing information representative of the current network topology. Typically, routing information is disseminated among interconnected intermediate network BGP nodes through advertising BGP update messages, or “BGP advertisements.” As used herein, a BGP advertisement generally describes any message used by a BGP routing protocol for communicating routing information among interconnected BGP nodes, i.e., routers and switches. Operationally, a remote BGP node (e.g., of a remote domain) may establish a BGP session with a local BGP node (e.g., of a local domain), and transmit a generated BGP advertisement to the local BGP node. Thereafter, the local BGP node may receive the transmitted BGP advertisement and update its routing table based on routing information contained in the received BGP advertisement. Notably, a BGP session between a local and remote domain (interdomain) is an external BGP (eBGP) session. The local BGP node may then transmit the received BGP advertisement to other BGP nodes of the local domain until each interconnected BGP node of the local domain has received the BGP advertisement and updated its local routing table. Notably, a BGP session within a domain (intradomain) is an internal BGP (iBGP) session. BGP nodes within a domain, such as an AS, are typically connected via a fully meshed iBGP session arrangement to ensure that all BGP nodes receive advertisements from the other BGP nodes in the AS. Notably, eBGP and iBGP are generally referred to herein as “BGP.”
In practice, each BGP node typically generates and disseminates a BGP advertisement whose routing information includes, inter alia, the set of domains (ASes) that must be traversed in order to reach certain destination address prefixes in remote domains. Particularly, the set of domains is generally transmitted as either an ordered or an unordered set of ASes within the BGP advertisement. When a BGP node advertises a prefix within its domain, the originating BGP node inserts an identification of the remote domain (an AS number) into the BGP advertisement. Prior to forwarding the BGP advertisement, a BGP node of each subsequent domain adds its AS number to the set of domains that must be traversed. In this way, each BGP node that receives a BGP advertisement for certain destination address prefixes learns the set of domains that must be traversed to reach the prefix in the origin domain, or the “domain route” to the prefix. Notably, when a BGP node receives advertisements from multiple domains that describe different domain routes to the same destination, the node chooses a single best domain route for reaching that destination (prefix). Once chosen, the node uses BGP to propagate that best domain route to its neighbors. The decision is based on the value of attributes, such as the number of domains to traverse, contained in a BGP advertisement and other BGP-configurable factors (e.g., policy).
Examples of an intradomain routing protocol, or an interior gateway protocol (IGP), are the Open Shortest Path First (OSPF) routing protocol and the Intermediate-System-to-Intermediate-System (IS-IS) routing protocol. The OSPF and IS-IS protocols are based on link-state technology and, therefore, are commonly referred to as link-state routing protocols. Link-state protocols define the manner with which routing information and network-topology information are exchanged and processed in a domain. This information is generally directed to an intradomain router's local state (e.g., the router's usable interfaces and reachable neighbors or adjacencies). The OSPF protocol is described in RFC 2328, entitled OSPF Version 2, dated April 1998 and the IS-IS protocol used in the context of IP is described in RFC 1195, entitled Use of OSI IS-IS for routing in TCP/IP and Dual Environments, dated December 1990, both of which are hereby incorporated by reference.
Link-state protocols are generally used to disseminate intradomain routing information representative of the current network topology of a particular domain (e.g., the local domain). For example, suppose the intermediate network node (IGP node) detects that one of its neighboring nodes (i.e., adjacent network nodes) becomes unavailable, e.g., due to a link failure or the neighboring node going “off-line,” etc. In this situation, the IGP node can update the routing information stored in its routing table to ensure that data packets are not routed to the unavailable network node. Furthermore, the IGP node also may communicate this change in network topology to the other IGP nodes so they, too, can update their local routing tables and bypass the unavailable node. In this manner, each of the IGP nodes becomes “aware” of the change in topology.
In practice, each IGP node typically generates and disseminates (“floods”) IGP advertisements to neighboring IGP nodes (e.g., of the local domain) in accordance with a predetermined network communication protocol, such as a link-state protocol (e.g., IS-IS, or OSPF). Conventional link-state protocols use link-state advertisements or link-state packets as the IGP advertisements, in which the routing information includes a list of the intermediate node's neighboring network nodes and one or more “cost” values associated with each neighbor. As used herein, a cost value associated with a neighboring node is an arbitrary metric used to determine the relative ease/burden of communicating with that node. For instance, the cost value may be measured in terms of the number of hops required to reach the neighboring node, the average time for a packet to reach the neighboring node, the amount of network traffic or available bandwidth over a communication link coupled to the neighboring node, etc.
IGP advertisements are usually flooded until each intermediate network IGP node has received an IGP advertisement from each of the other interconnected intermediate nodes. Notably, a BGP node may also be configured as an IGP node, such that certain information received via BGP advertisements (e.g., prefixes in remote domains) may be forwarded to IGP nodes in accordance with IGP advertisements. Then, each of the IGP nodes (e.g., in a link-state protocol) can construct the same “view” of the network topology by aggregating the received lists of neighboring nodes and cost values. To that end, each IGP node may input this received routing information to a “shortest path first” (SPF) calculation that determines the lowest-cost network paths that couple the intermediate node with each of the other network nodes. For example, the Dijkstra algorithm is a conventional technique for performing such an SPF calculation, as described in more detail in Section 12.2.4 of the text book Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein. Each IGP node updates the routing information stored in its local routing table based on the results of its SPF calculation. More specifically, the RIB updates the routing table to correlate destination nodes with next-hop interfaces associated with the lowest-cost paths to reach those nodes, as determined by the SPF calculation.
Multi-Protocol Label Switching (MPLS) Traffic Engineering has been developed to meet data networking requirements such as guaranteed available bandwidth or fast restoration. MPLS Traffic Engineering exploits modern label switching techniques to build guaranteed bandwidth end-to-end tunnels through an IP/MPLS network of label switched routers (LSRs). These tunnels are a type of label switched path (LSP) and thus are generally referred to as MPLS Traffic Engineering (TE) LSPs. Examples of MPLS TE can be found in RFC 3209, entitled RSVP-TE: Extensions to RSVP for LSP Tunnels dated December 2001, RFC 3784 entitled Intermediate-System-to-Intermediate-System (IS-IS) Extensions for Traffic Engineering (TE) dated June 2004, and RFC 3630, entitled Traffic Engineering (TE) Extensions to OSPF Version 2 dated September 2003, the contents of all of which are hereby incorporated by reference in their entirety.
Establishment of an MPLS TE-LSP from a head-end LSR to a tail-end LSR involves computation of a path through a network of LSRs. Optimally, the computed path is the “shortest” path, as measured in some metric, that satisfies all relevant LSP Traffic Engineering constraints such as e.g., required bandwidth, “affinities” (administrative constraints to avoid or include certain links), etc. Path computation can either be performed by the head-end LSR or by some other entity operating as a path computation element (PCE) not co-located on the head-end LSR. The head-end LSR (or a PCE) exploits its knowledge of network topology and resources available on each link to perform the path computation according to the LSP Traffic Engineering constraints. Various path computation methodologies are available including CSPF (constrained shortest path first). MPLS TE-LSPs can be configured within a single domain, e.g., area, level, or AS, or may also span multiple domains, e.g., areas, levels, or ASes.
The PCE is an entity having the capability to compute paths between any nodes of which the PCE is aware in an AS or area. PCEs are especially useful in that they are more cognizant of network traffic and path selection within their AS or area, and thus may be used for more optimal path computation. A head-end LSR may further operate as a path computation client (PCC) configured to send a path computation request to the PCE, and receive a response with the computed path, which potentially takes into consideration other path computation requests from other PCCs. It is important to note that when one PCE sends a request to another PCE, it acts as a PCC. PCEs conventionally have limited or no visibility outside of its surrounding area(s), level(s), or AS. A PCC can be informed of a PCE either by pre-configuration by an administrator, or by a PCE Discovery (PCED) message (“advertisement”), which is sent from the PCE within its area or level or across the entire AS to advertise its services.
One difficulty that arises in crossing domain boundaries is that path computation at the head-end LSR requires knowledge of network topology and resources across the entire network between the head-end and the tail-end LSRs. Yet service providers typically do not share this information with each other across domain borders. In particular, network topology and resource information do not generally flow across area boundaries even though a single service provider may operate all the areas. Neither the head-end LSR nor any single PCE will have sufficient knowledge to compute a path where the LSR or PCE may not have the required knowledge should the destination not reside in a directly attached domain. Because of this, MPLS Traffic Engineering path computation techniques are required to compute inter-domain TE-LSPs.
In order to extend MPLS TE-LSPs across domain boundaries, the use of PCEs may be configured as a distributed system, where multiple PCEs collaborate to compute an end-to-end path (also referred to as “Multi-PCE path computation”). An example of such a distributed PCE architecture is described in commonly-owned copending U.S. patent application Ser. No. 10/767,574, entitled COMPUTING INTER-AUTONOMOUS SYSTEM MPLS TRAFFIC ENGINEERING LSP PATHS, filed by Vasseur et al., on Sep. 18, 2003, the contents of which are hereby incorporated by reference in its entirety. In a distributed PCE architecture, the visibility needed to compute paths is extended between adjacent domains so that PCEs may cooperate to compute paths across multiple domains by exchanging virtual shortest path trees (VSPTs) while preserving confidentiality across domains (e.g., when applicable to ASes).
Some applications may incorporate unidirectional data flows configured to transfer time-sensitive traffic from a source (sender) in a computer network to a destination (receiver) in the network in accordance with a certain “quality of service” (QoS). Here, network resources may be reserved for the unidirectional flow to ensure that the QoS associated with the data flow is maintained. The Resource ReSerVation Protocol (RSVP) is a network-control protocol that enables applications to reserve resources in order to obtain special QoS for their data flows. RSVP works in conjunction with routing protocols to, e.g., reserve resources for a data flow in a computer network in order to establish a level of QoS required by the data flow. RSVP is defined in R. Braden, et al., Resource ReSerVation Protocol (RSVP), RFC 2205. In the case of traffic engineering applications, RSVP signaling is used to establish a TE-LSP and to convey various TE-LSP attributes to routers, such as border routers, along the TE-LSP obeying the set of required constraints whose path may have been computed by various means.
Because a BGP node may advertise the best domain route based on certain attributes, such as the number of domains to traverse to the destination or policy, PCE-based path computation techniques for TE-LSPs across multiple domains are generally limited to traversing the “best” domain route based on BGP standards. Consequently, path computation techniques currently rely on the assumption of tree-based domain structures (one known domain route for each destination prefix) or a manually configured set of domains to traverse (e.g., by an administrator with knowledge of desired domain routes). However, in certain circumstance, the “best” path for a given TE-LSP from a source to a destination, e.g., based on different cost metrics, may be through a different domain route than that advertised through BGP. There remains a need, therefore, for a technique that efficiently determines the set of domain routes to the tail-end node of a TE-LSP, and generates path computation requests for each possible domain route in order to select the shortest path from a source to a destination based on TE-LSP metrics.